Furnace for producing oriented graphite



Jan. 11, 1966 1.. PODOLSKY 3,228,373

FURNACE FOR PRODUCING ORIENTED GRAPHITE Original Filed May 28, 1962 INVENTOR: E/V Pa as/(r United States Patent Ofiice 3,228,373 Patented Jan. 11, 1966 3,228,373 FURNACE FOR PRODUCHNG GRlENTED GRAlHiTE Leon Podolsky, Pitts-field, Mesa, assignor to Drexel Institute of Technology, a corporation of Pennsylvania Original application May 28, 1962, Ser. No. 198,286, new

Patent No. 3,142,158, dated July 28, 1964. Divided and this application Mar. 26, N64, er. N 354,923

1 Claim. (Cl. 113-495) This invention relates to the deposition of graphite films, and more particularly to the production of oriented graphite. This application is a division of co-pending application Serial No. 198,286, filed May 28, 1962, now US. Patent No. 3,142,158.

The term oriented graphite is used herein to mean graphite, the crystals of which are all, or substantially all, oriented with their c axes in a single direction.

It is a general object of the present invention to provide a novel furnace for producing oriented graphite.

It is another object of the invention to provide a furnace in which the surface of a body can be coated with a film of oriented graphite.

It is a more specific object of the invention to provide a furnace in which oriented graphite can be deposited on a metal tube.

Oriented graphite produced in the furnace of the present invention has many potential uses. For the sake of illustration, and to render the invention completely understandable, one purpose for which the present furnace has been found particularly advantageous will be referred to hereinafter. It is in connection with the making of a thermoelectric cooling device such as that described in the above-mentioned co-pending application.

In that application, a cooling device is described in which passage of an electric current through a thermocouple is caused to produce the desired elfect. It is of course well known that when two dissimilar electrically conductive materials are placed in contact thereby forming a thermocouple, and an electric current is passed through them in a certain direction, a reduction in temperature takes place at the junction between the two materials.

In the device mentioned above, a metal tube serves as the self-sustaining member of a thermocouple, and the tube carries on its outer surface an electrically conductive graphite film. The tube and film form a thermocouple which, when connected in series to a source of electric current, becomes cooled. If the tube is employed as part of a fluid circuit, the fluid moving through the circuit is cooled as it flows through the tube.

As discussed in applicants co-pending application, Serial No. 195,681, filed May 18, 1962, in order that the graphite have a maximum figure of merit the crystals of the graphite film must have a particular orientation with respect to the flow of electric current through the graphite. As mentioned above, the present invention provides a thoroughly practical furnace by means of which the graphite crystal orientation may be accomplished.

Other objects and advantages of the invention will be apparent from the following detailed description in which reference is made of the accompanying drawings.

In the drawings:

FIG. 1 is a perspective view of a metal tube;

FIG. 2 is a cross-sectional view of a furnace according to this invention for depositing an oriented graphite film on tube such as the one shown in FIG. 1; and

FIG. 3 is a view similar to FIG. 1 showing the tube carrying a graphite film on its outer surface.

The present invention is illustrated in connection with a tubular thermoelectric cooling device. However, it is to be understood that the usefulness of the invention is not limited to cooling devices or to tubular surfaces.

As is well known, the performance characteristics of thermoelectric devices can be conveniently rated by a value known as the Figure of Merit. This parameter is proportional to the square of the thermoelectric E.M.F., and inversely proportional to resistivity and thermal conductivity of the couple, all measured in the direction of current fiow through the junction of the thermocouple, and the Figure of Merit is advantageously as high as possible. Graphite does not exhibit a high enough Figure of Merit for practical purposes unless deposited on the tube in such a way that its crystals have a particular orientation mentioned below.

The metal tube ltl may be provided with a graphite coating 11 in a deposition furnace 12 (FIG. 2) illustrative of the present invention. Deposition furnaces are well known and may include a cylindrical shell 13 of insulating material into which is introduced, as by means 8, a hydrocarbon gas, such as methane, and which is heated, as by electric means 9, to a temperature (of the order of 2100 F.) capable of burning and cracking the hydrocarbon gas. According to the present invention the shell 13 of the furnace is girdled by an outer tubular electrode 14. A metal rod 15 serving as an inner electrode extends longitudinally through the furnace. The rod is appropriately formed to carry the metal tube or tubes it? to be coated. Throughout the cracking operation which causes crystalline graphite to be deposited on the relatively cool surfaces of the tubes 10, a high direct current potential difference is applied between the electrodes 14 and 15 producing an intense electrostatic field within the furnace. An electrostatic field of several thousand volts per inch is usually employed. The direction of this field will obviously be perpendicular to every point on the outer surface of each tube 10, and the effect of the field will be to cause the graphite crystals to be deposited with their 0 axes in the direction of the field, i.e., the c axes of the deposited crystals will be perpendicular to the surface 'of the tube upon which they are deposited.

A graphite crystal exhibits along its 0 axis its highest thermoelectric E.M.F. and its lowest thermoconductivity (both favorable factors as far as Figure of Merit is concerned). However, the resistivity along this axis is highest, a factor which normally tends to reduce the Figure of Merit. Nevertheless, when the graphite is employed in the form of a thin film or coating, and the direction of current flow is transversely through the film, the resistivity is negligible even though the current flow is along the c axis.

In the drawings, the thickness of the film has been greatly exaggerated for clarity. In practice, the film is ordinarily less than twenty microns in thickness.

The tube it) and film 11 are, of course, a thermocouple, and if the film is connected to one terminal of an electrical source and the tube to the other terminal, the tube and film experience a reduction in temperature.

The invention has been shown and described in terms of a furnace for coating a metal tube with a film of graphite, the crystals of which are oriented with their c axes perpendicular to the surface of the tube. However, it is obvious that the invention has much broader significance with respect to production of oriented graphite in general. It is understod, therefore, that the invention is not limited to any specific form or embodiment except insofar as such limitations appear in the appended claim.

What is claimed is: A furnace for depositing oriented graphite on tubes comprising a tubular shell of insulating material, a tubular electrode surrounding the outer surface of said shell, a metal rod extending longitudinally through said shell adapted to carry the tubes, means for heating the interior of said shell to a temperature sufficient to crack a hydrocarbon gas, means for supplying a hydrocarbon gas to the interior of said shell, and means for applying a potential ditference between said electrode and said rod in order to create an electrostatic field within said shell serving to orient the graphite crystals as they are deposited on the surfaces of the tubes so that their 0 axes are perpendicular to said surfaces of the tubes.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,231 10/1956 Matejka 117-106 5 2,685,535 8/1954 Nack 11849.5 X 2,788,296 4/1957 Louis 117106 3,096,213 7/ 1963 McCurtain 1l8-64 X CHARLES A. WILLMUTH, Primary Examiner.

10 W'ILLIAM D. MARTIN, WILLIAM B. PENN,

Examiners. 

